Castor is one of two bright stars in the constellation Gemini the Twins. It appears as a single star, but it’s actually a famous multiple star system, containing three pairs of binary stars all revolving in a complex way around a common center of mass. In other words, the single bright light we see as Castor is really six stars in one. Follow the links below to learn more about Castor, one of Gemini’s two bright stars.

Pollux on left. Castor on right. See the contrast in their colors? Pollux is golden, and Castor is white. Image via Warren Wilson College

Gemini the Twins

How to see the star Castor. The single point of light we call Castor appears as one of two bright stars in the constellation Gemini the Twins. It appears as a prominent white star, noticeable for its nearness to its brother star, Pollux in Gemini. No other two such bright stars appear so close together on our sky’s dome.

Castor and Pollux are sometimes called the “twin” stars, but you can tell them apart easily. The brighter and more golden star is Pollux. The fainter, whiter star is Castor.

These two stars of nearly equal brightness in Earth’s sky are not really related in space, and Pollux is not one of the six stars in the Castor system. At 34 light-years, Pollux is closer than Castor (52 light-years). Thus Pollux and Castor aren’t gravitationally bound, but only near each other along our line of sight. Plus they are different kinds of stars. But their proximity in our sky makes them easy to spot.

Castor and Pollux are well placed in the evening sky from late November through May. Castor is opposite the sun in middle January. In February, Castor is particularly well placed for evening viewing. It is well up in the east an hour after sunset in late February – high in the sky throughout the evening hours.

Castor and Pollux are close to the moon’s path across our sky. The moon passes no more than 15 and sometimes fewer than 5 degrees south of Castor on some day every month. For this reason, the moon might help you find Castor.

The sun passes closest to Castor on or near July 14, and consequently the star cannot be seen for several weeks before that date to several weeks afterward. Other than for that time period, intrepid observers can find Castor at some time of night for about 10 months of the year.

Castor is really six stars in one. Even a fairly small telescope shows that Castor appears double, and in fact an even fainter star nearby also is part of the same system.

Each of these three stars (Castor A, B and C) is also double. This is not directly visible in telescopes but obvious through the use of a spectroscope, which is a device for splitting starlight into its component colors. The two larger and visible components in the Castor system – showing in the drawing on this page – are hot A-type stars, whereas the smaller ones are cool, M-type red dwarf stars.

The mass of all six stars together is, very roughly, about six times that of the sun. So, like our sun, they’re a bit on the puny side, as stars go.

History and mythology of Castor. Castor is designated Alpha Geminorum. Most alpha stars are the brightest in their constellations, but Castor is slightly fainter than Pollux.

The reason for the name Castor is unclear, although there appears no specific connection with a beaver, which is what the word means in Latin. It’s also not clear why this star holds the appellation of alpha, a label typically put on a constellation’s brightest star. Castor is clearly secondary in brightness to Pollux, which is officially called Beta Geminorum.

There is much mythology associated with these two stars, typically only in conjunction with each other. They are usually considered to be twins. In Greek mythology Pollux is immortal, the son of Zeus, and Castor is mortal, the son of King Tyndareus of Sparta. Thus they were really half-brothers rather than true twins, with a common mother in Queen Leda. Their conception and birth, however, was a complicated and unlikely affair, with their mother succumbing to both Zeus (disguised as a swan) and King Tyndareus on the same night, with the resulting birth not only of Castor and Pollux, but of their sister Helen of Troy. Castor and Pollux later were among the argonauts who sailed with Jason in search of the Golden Fleece, and due to their mutual devotion, Zeus placed them both in the heavens on their death, so that they could remain together forever.

Although many cultures saw Castor and Pollux as twins, early Christians sometimes called them David and Jonathan, whereas the Arabs knew them as peacocks. Perhaps the most unexpected connotation for the twins (along with the rest of Gemini) was as a “pile of bricks” as reported by Richard Hinckley Allen. Apparently the pile of bricks stood for the foundation of Rome, and in that context Castor and Pollux were associated with Romulus and Remus, the city’s legendary twin founders.

Castor’s position is RA: 07h 34m 36s, dec: +31° 53′ 19″

Bottom line: The star Castor, which appears as one of two bright stars in the constellation Gemini the Twins, is actually a multiple star system.

Larry Sessions has written many favorite posts in EarthSky's Tonight area. He's a former planetarium director in Little Rock, Fort Worth and Denver and an adjunct faculty member at Metropolitan State University of Denver. He's a longtime member of NASA's Solar System Ambassadors program. His articles have appeared in numerous publications including Space.com, Sky & Telescope, Astronomy and Rolling Stone. His small book on world star lore, Constellations, was published by Running Press.